A circular body (54), having a throughgoing orifice (56), an externally threaded periphery (60), and a pair of stepped recesses (62,64), defines a valve seat for a reciprocating piston (78,80). A retaining ring (72) is made fast in one of the recesses (64), and a seal (74) which contracts upon cooling to the ambient, after having been exposed to elevated temperatures, is set in the other recess (62). The seal (74) is formed of a composite of plastic in which are dispersed carbon fibers to render the seal resistant to creep. A housing (70) having a pair of chambers formed therein has the orificed body and piston interposed between the chambers for valvingly controlling inter-chamber communication.